Sócrates resigned as General Secretary on election night of 5 June 2011, due to having lost the general election by a margin higher than expected. Seguro was elected as leader of the party on 23 July, winning 68% of the vote against his challenger, Francisco Assis, who got 32%.

The Socialist Party (PS) was created at a conference of Portuguese Socialist Action (ASP), at that time in exile, on 19 April 1973, in Bad Münstereifel in West Germany. The twenty-seven delegates decided to found a party of socialism and freedom, making an explicit reference to a classless society and without Marxism, redesigned as a source of principal inspiration.

On 25 April 1974, the Carnation Revolution brought down the authoritarian regime of the Estado Novo, established in 1933, and democracy was restored. The general secretary of the PS, Mário Soares, returned to Portugal after being in exile in France, and became Minister of Foreign Affairs, and António de Almeida Santos was appointed Minister of Interjurisdictional Coordination in one of the first provisional governments.

In 1995, the Socialist Party, then led by António Guterres, won a general election for the first time in twelve years, and in 1999, they failed to obtain what would have been an historic absolute majority for the party by only one MP. In 2001, after a massive defeat in the 2001 local elections, António Guterres resigned as Prime Minister and called for new elections in 2002. The Socialist Party lost the 2002 general election by a small margin to the PSD, who formed a coalition government with the People's Party (CDS–PP).

In June 2004, the PS won the 2004 European elections by a landslide, and a few weeks later, Durão Barroso, leader of the PSD and Prime Minister, resigned to become President of the European Commission. In December 2004, Jorge Sampaio, President of the Republic, called fresh elections for February 2005. These elections resulted in a landslide victory for the PS, winning for the first time since its foundation an absolute majority. José Sócrates, leader of the PS, became Prime Minister.

In the elections, the PS suffered a huge setback, with 28.1% of the vote, ten points behind the PSD, who formed another coalition government with the CDS-PP. Sócrates resigned as General Secretary on election night after the PS's worst result since 1987. On 23 July 2011, António José Seguro was elected as Sócrates' successor.

The PS, under the leadership of Seguro, won the 2013 local elections making significant gains over the PSD and the Socialists again won the European elections on May 2014 but this time only just. They won 31.5% of the vote against the almost 28% of the alliance between the PSD and CDS-PP. The result was considered quite a disappointment to many Socialist party members and supporters and on May 27 António Costa, mayor of Lisbon, announces that he will stood for the leadership of the Socialist Party.[7] António José Seguro refused to call a new congress and leadership election and instead called for a primary election, to be held on 28 September, to elect the party's candidate for Prime Minister for the 2015 general elections.[8]